Carrier Conversion Research Articles

Tuning the Structural and Optoelectronic Properties of Cs2 AgBiBr6 Double-Perovskite Single Crystals through Alkali-Metal Substitution.

Lead-free double perovskites have great potential as stable and nontoxic optoelectronic materials. Recently, Cs2 AgBiBr6 has emerged as a promising material, with suboptimal photon-to-charge carrier conversion efficiency, yet well suited for high-energy photon-detection applications. Here, the optoelectronic and structural properties of pure Cs2 AgBiBr6 and alkali-metal-substituted (Cs1- x Yx )2 AgBiBr6 (Y: Rb+ , K+ , Na+ ; x= 0.02) single crystals are investigated. Strikingly, alkali-substitution entails a tunability to the material system in its response to X-rays and structural properties that is most strongly revealed in Rb-substituted compounds whose X-ray sensitivity outperforms other double-perovskite-based devices reported. While the fundamental nature and magnitude of the bandgap remains unchanged, the alkali-substituted materials exhibit a threefold boost in their fundamental carrier recombination lifetime at room temperature. Moreover, an enhanced electron-acoustic phonon scattering is found compared to Cs2 AgBiBr6 . The study thus paves the way for employing cation substitution to tune the properties of double perovskites toward a new material platform for optoelectronics.

Experimental analysis of biomass to biodiesel conversion using a novel renewable combined cycle system

Abstract In this study, a novel renewable hybrid system was designed and constructed to efficiently produce biodiesel through various methods. First, a parabolic trough solar collector (PTC) was used as a renewable source for producing desalinated water in a solar still. Then, the heat dissipated by PTC and water produced by the solar still were used to generate biodiesel. Finally, the combined cycle of PTC/desalination/biodiesel generator was applied to produce heat and steady power. Necessary electricity in the hybrid system containing pumps, mixing tools, etc. was provided by photovoltaic panels. After launching the hybrid system and performing experiments to produce biodiesel on Palm and Chlorella bioenergy carriers, the percentage of the biodiesel production for these two species in different ways were as follows: alkaline catalyzed transesterification (79.17,81.46), acid-catalyzed transesterification (79.1,81.43), two-step transesterification (78.07,81.42), supercritical transesterification (75.16,80.68), microemulsion (76.3,81.4), electrolysis (50.25, 58.87), and pyrolysis (13.677,14.833). Moreover, the lowest price belonged to the alkaline catalyzed transesterification which for palm/chlorella was 0.65/0.972. In most methods, the cetane number of palm-based biodiesel was higher (∼58). Consequently, it had a better combustion quality compared to chlorella-based biodiesel (∼44). This study also modeled various processes for the bioenergy carrier conversion. The results were consistent with experimental data.

The competition between direct gas–solid reduction and oxygen uncoupling of CuO oxygen carrier in chemical looping with oxygen uncoupling: A single particle simulation study

In chemical looping with oxygen uncoupling (CLOU), the oxygen carrier (e.g., CuO-based materials) can be reduced either by direct decomposition or by heterogeneous gas–solid reaction. In most cases, the above two reaction pathways take place in parallel and compete with each other, which therefore make it very difficult to quantify the exact proportion of their contributions to the oxygen carrier conversion. The failure to distinguish the individual significance of each reaction route hinders the acquisition of a convincing reaction kinetic model. In this work, the conversion processes of CuO oxygen carrier in free space and in thermogravimetric crucible were simulated by a single particle model, in which the heterogeneous/homogeneous reactions as well as the heat/mass transfer inside and outside a porous particle were incorporated. The competition between the direct gas–solid reduction (using H2 as fuel gas) and oxygen uncoupling of the CuO oxygen carrier was analyzed from the aspects of the relative significance of each reaction route and the interactions between them. The results showed that the significances of each reaction route varied with the temperature and H2 concentration conditions. To quantify the relative significance of each reaction route at different conditions, the controlling regime diagram of CuO conversion in free space was given. The CuO conversion was more likely to be dominated by the oxygen uncoupling process when the particle was in crucible. For the conversion of CuO in free space, the exothermic gas–solid reduction increased the particle temperature and further facilitated the oxygen uncoupling rate, which was considered as the predominant interactions. Differently, for the conversion of CuO in crucible, the key interactions were characterized as: the homogeneous reaction between H2 and O2 decreased the O2 concentration neighboring the CuO particle and thus increased its oxygen uncoupling rate.

Hydrocarbon hydrogen carriers for catalytic transfer hydrogenation of guaiacol

Polycyclic hydrocarbons are known to be efficient hydrogen carriers capable of yielding high purity H2 upon dehydrogenation. Due to their high hydrogen density, high boiling point, and stability, these compounds demonstrate the potential to be used as hydrogen donors under catalytic transfer hydrogenation (CTH) conditions. In this work, the potential of a suite of hydrogen carriers to donate hydrogen, as well as the mechanisms affecting their hydrogen transfer, are assessed through the CTH of guaiacol, on Pd/Al2O3, as a model system. The results indicated the following descending order of transfer hydrogenation rate: bicyclohexyl > tetralin » hydrogenated terphenyl (HTP) > cyclohexylbenzene. Among the products, cyclohexanone and phenol are the most abundant, directly resulting from CTH. Detailed analysis of the hydrogen carrier conversion and selectivity clearly shows that the potential for CTH is highly linked to the molecular structure of the donor, rather than the amount of hydrogen available for transfer. A density functional theory (DFT) study, supported by experimental data, reveals that when unsaturated hydrocarbons are utilized, such as tetralin, cyclohexylbenzene, and HTP, the effective CTH rate to guaiacol is limited, despite dehydrogenation being more favorable for those molecules than from fully saturated donors, such as bicyclohexyl.

Hybrid FeNiOOH/α-Fe2O3/Graphene Photoelectrodes with Advanced Water Oxidation Performance.

In this study, the photoelectrochemical behavior of electrodeposited FeNiOOH/Fe2O3/graphene nanohybrid electrodes is investigated, which has precisely controlled structure and composition. The photoelectrode assembly is designed in a bioinspired manner where each component has its own function: Fe2O3 is responsible for the absorption of light, the graphene framework for proper charge carrier transport, while the FeNiOOH overlayer for facile water oxidation. The effect of each component on the photoelectrochemical behavior is studied by linear sweep photovoltammetry, incident photon‐to‐charge carrier conversion efficiency measurements, and long‐term photoelectrolysis. 2.6 times higher photocurrents are obtained for the best‐performing FeNiOOH/Fe2O3/graphene system compared to its pristine Fe2O3 counterpart. Transient absorption spectroscopy measurements reveal an increased hole‐lifetime in the case of the Fe2O3/graphene samples. Long‐term photoelectrolysis measurements in combination with Raman spectroscopy, however, prove that the underlying nanocarbon framework is corroded by the photogenerated holes. This issue is tackled by the electrodeposition of a thin FeNiOOH overlayer, which rapidly accepts the photogenerated holes from Fe2O3, thus eliminating the pathway leading to the corrosion of graphene.

Synergistic effects of plasma Z-scheme photocatalysis process for biogas conversion

Biogas can be an energy carrier and on-site conversion of biogas into syngas provides an alternative to reduce its utilization cost and to increase its energy density since CO2 can be converted into combustible contents. In this study, perovskite-type photocatalyst LaFeO3 (LFO) and Ag-LaFeO3/TiO2 (ALFTO) are prepared and combined with a spark discharge reactor to conduct biogas reforming. Next, to reduce the influence of H2S on reforming system, another photocatalyst BaTa0.7Nb0.3O3 (BTaNO) is added to form plasma Z-scheme photocatalysis system. Results indicate that packing ALFTO enhances syngas generation rate to 20.2 mol/kW h but the existence of H2S leads to deactivation of photocatalyst and restriction on discharge. Further introduction of BTaNO increases the stable discharge time of the reactor. Photocatalyst characterizations further indicate that there are interactions between plasma and photocatalyst to induce synergistic effects and Z-scheme photocatalysis is beneficial to alleviate the impact of H2S on biogas reforming.

Investigations on morphology and thermoelectric transport properties of Cu+ ion implanted bismuth telluride thin film

Present study reports the effect of copper (Cu+) ion implantation (with different Cu concentrations of 0.5 at%, 1 at%, 1.5 at%, and 2 at%) on thermoelectric transport properties of bismuth telluride thin films. The field emission-scanning electron microscopy and atomic force microscopy results reveal the coalescence of columnar grains with the formation of pinholes in the Cu+ ion implanted samples and the evolution of morphology was explained successfully in the framework of spherical thermal spike model. X-ray diffraction and Raman scattering experiments revealed the reduction in the crystalline nature of Bi2Te3 thin films upon Cu+ ion implantation. The depth profiles for the atomic concentration of Bi, Te, and Cu were determined using Rutherford backscattering spectrometry. As a function of Cu+ ion fluences, the Bi atomic concentration gradually decreases across the Bi2Te3 thin films. The Hall-effect and Seebeck coefficient measurements demonstrate the n-type to p-type charge carrier conversion upon ion implantation. The charge carrier conversion is attributed to the high density of defect complexes in addition to implanted Cu atoms. These results show the possibility of fabricating TE micro-modules that comprises of n- and p-type semiconductors using ion implantation.

Radiation detection and energy conversion in nuclear reactor environments by hybrid photovoltaic perovskites

Detection and direct power conversion of high energy and high intensity ionizing radiation could be a key element in next generation nuclear reactor safety systems and space-born devices. For example, the Fukushima catastrophe in 2011 could have been largely prevented if 1% of the reactor’s remnant radiation (γ-rays of the nuclear fission) were directly converted within the reactor to electricity to power the water cooling circuit. It is reported here that the hybrid halide perovskite methylammonium lead triiodide could perfectly play the role of a converter. Single crystals were irradiated by a typical shut-down γ-spectrum of a nuclear reactor with 7.61 × 1014 Bq activity exhibit a high-efficiency of γ-ray to free charge carrier conversion with radiation hardening. The power density of 0.3 mW/kg of methylammonium lead triiodide at 50 Sv/h means a four times higher efficiency than that for silicon-based cells. The material was stable to the limits of the experiment without changing its performance up to 100 Sv/h dose rate and 57 Sv H*(10) ambient total γ-dose. Moreover, the γ-shielding performance of methylammonium lead triiodide was found to be superior to both ordinary and barite concrete.

Hot carrier dynamics in a dispersionless plasmonic system

Hot carrier dynamics in a broadband, dispersionless, plasmonic structure are studied using pump-probe measurements with 45 fs time resolution. The results obtained by simultaneous measurement of differential transmittance and reflectance show the influence of pump generated carriers on the probe generated plasmons. The dependence of decay time on the pump fluence is linear for the interband and intraband carrier excitation near the L-symmetry point in gold. However, 1.908 eV pump excitation, which allows both interband transition at the X-symmetry point and intraband excitation at the L-symmetry point, shows quadratic dependence of decay time on the pump fluence. Degenerate pump-probe measurements at 1.55 eV, which allow L-valley intraband excitations, highlight the difference in surface (reflection) and the bulk (transmission) mechanisms. Hot carrier relaxation time is in the 1–3 ps range for different excitation energies. Rise time, governed by the plasmon to hot carrier conversion and electron-electron scattering processes, is about 200 ± 35 fs for the hot carrier and hot plasmon excitation cases that increased to about 485 ± 35 fs when the pump is resonant with the interband transition at the X-symmetry point. Results show that the hot carrier dynamics are governed by the bulk metal band structure. The dipole matrix element for each of the transitions is estimated by density matrix calculations.

Stable and sustainable photoanodes using zinc oxide and cobalt oxide chemically gradient nanostructures for water-splitting applications

Amorphous cobalt oxide (CoO) encapsulated zinc oxide (ZnO) nanostructures were developed by adopting three low-temperature methods respectively atomic layer deposition, chemical bath deposition, and electrochemical deposition. The impact of CoO growth on the physical and chemical properties of ZnO nanostructures was investigated. Then, the ZnO/CoO core/shell nanostructures grown under optimized conditions were adopted for the fabrication of photoelectrochemical (PEC) water-splitting devices. The catalytic performance of ZnO nanostructures is substantially improved after their encapsulation with CoO layers. In addition, the chemical stability and durability of the structures are significantly enhanced. Under typical measurement conditions, these surface-modified ZnO nanostructures exhibited incident photon to charge carrier conversion efficiency (IPCE) higher than 16%, and a stable photocurrent density of 1.25 mA cm−2. Further, these ZnO/CoO nanostructured photoanodes showed a high illumination to dark current density ratio, ~2910.

An efficient wide range photodetector fabricated using a bilayer Bi2S3/SnS heterojunction thin film

This study presents a detailed optoelectronic investigation on the bilayer Bi2S3/SnS heterojunction photodetector. The Bi2S3 and SnS nanostructures were synthesized by an ultrasonication method and deposited in the form of thin films by a thermal evaporation system to fabricate the photodetectors. The performed optoelectrical measurements showed that the Bi2S3 and Bi2S3/SnS photodetectors are respectively n- and p-type electrical conductivity. This finding was further confirmed by current–voltage (I–V) analysis, Mott–Schottky plots and photoresponse measurements. According to the obtained gain and sensitivity values, the Bi2S3/SnS heterojunction provides a higher photon-to-electrical carrier conversion relative to the Bi2S3 photodetectord due to the enhanced photogeneration of electron–hole pairs and separation of the photoinduced charge carriers under the influence of the electric field build-in at the heterojunction’s interface. The heterojunction exhibited a remarkable photoresponse from 400 to 800 nm by giving the photocurrent gain (G), sensitivity (S), and external quantum efficiency (EQE) values of 8460%, 450% and 45%, respectively. The obtained results represent the Bi2S3/SnS heterojunction photodetector as a potential candidate for devising high-performance optoelectronic devices.

Electrocatalytic glycerol oxidation enabled by surface plasmon polariton-induced hot carriers in Kretschmann configuration.

Plasmonic hot carrier generation has attracted increasing attention due to its ability to convert light to electrical energy. The generation of plasmon-induced hot carriers can be achieved via Landau damping in the non-radiative decay process of the plasmonic excitation energy. Localized surface plasmons (LSPs) undergo both radiative and non-radiative decays, while surface plasmon polaritons (SPPs) dissipate only via the non-radiative decay. Thus, it is a challenging task to exploit the surface plasmon polaritons for the efficient generation of hot carriers and their applications. In this study, a model hot-carrier-mediated electrocatalytic conversion system was demonstrated using an Au thin film in Kretschmann configuration, which is the representative platform to excite SPPs. AgPt-decorated Au nanobipyramids (AuNBPs) were designed and introduced onto the Au film, creating hot-spots to revolutionize the thin film-based photon-to-carrier conversion efficiency. The glycerol electro-oxidation reaction enabled by such SPP-induced hot carriers was evaluated and exhibited a photon-to-hot carrier conversion efficiency of 2.4 × 10-3%, which is ∼2.5 times enhanced as compared to the efficiency based on the neat Au film.

Electron emission and capture by oxygen-related bistable thermal double donors in silicon studied with junction capacitance techniques

It has been recently suggested that oxygen-related bistable thermal double donors (BTDDs) are responsible for the reduction of minority carrier lifetime and conversion efficiency of novel amorphous-crystalline Si heterojunction solar cells with their base formed from n-type Czochralski-grown (Cz) silicon [M. Tomassini et al., J. Appl. Phys. 119, 084508 (2016)]. To test this hypothesis, we have studied processes associated with carrier emission and capture by BTDDs in p+-n and Schottky barrier diodes on n-type Cz-Si materials with the use of junction capacitance techniques. By means of deep level transient spectroscopy, we have detected electron emission signals from the deep donor state of the BTDD-0 and BTDD-1 centers. The values of activation energy for electron emission (Eem) have been determined as 1.01 ± 0.01 and 0.91 ± 0.01 eV for the BTDD-0 and BTDD-1 centers, respectively. Such high Eem values are very unusual for defects in Si. We have carried out measurements of electron capture kinetics and associated shallow donor–deep donor transformations for the BTDD-0 and BTDD-1 defects at different temperatures in the diodes with different doping levels. Energy barriers for the capture-transformation processes have been determined. It is argued that BTDDs are responsible for carrier trapping in n-type Cz-Si crystals but are not effective recombination centers.

High photoresponsivity and light-induced carrier conversion in RGO/TSCuPc hybrid phototransistors

Abstract

250 kWth high pressure pilot demonstration of the syngas chemical looping system for high purity H2 production with CO2 capture

Chemical looping combustion (CLC) is an advanced technology for converting fossil fuel while achieving in-situ CO2 capture. The high purity CO generated in the combustion product stream is sequestration/utilization ready, which makes chemical looping one of the most attractive carbon emission control technology. In this paper, the authors present the design, simulation and experimental operation results of the 250 kWth high pressure syngas chemical looping (SCL) pilot plant. The pilot plant’s unique countercurrent moving bed design allows near-full conversion of coal-derived syngas with simultaneous production of high purity, carbon-free H2. Critical aspects of the design efforts, including heat and material balances, reactor sizing approach and solid flow control and measurement devices are presented. An ASPEN Plus® model is constructed to predict the gas and solid conversions of the SCL process the gas and solid conversions of the SCL process under different experimental conditions. The highest syngas conversion achieved was 97.95% with 16.03% oxygen carrier conversion, which were close to the thermodynamic limits for both the gas and solid phases in the reducer. Differences between the experimental results and predicted conversion values were more significant under conditions with lower oxygen carrier to fuel ratio. Greater than 99% purity H2 was produced from the moving bed oxidizer, and the scalability and feasibility of SCL process were successfully demonstrated.

Oxidation kinetics of YBaCo4O7+δ and substituted oxygen carriers.

In this paper, the relaxation kinetics of the oxidation process of the YBaCo4O7+δ, Y0.95Ti0.05BaCo4O7+δ and Y0.5Dy0.5BaCo4O7+δ oxygen carriers is studied with isothermal reaction data. XRD analysis for fresh samples shows that all the samples have YBaCo4O7+δ structure. Scanning electron microscopy images of samples show that the samples consist of porous agglomerates of primary particles. Isothermal TG experiments are conducted with temperatures of 290°C, 310°C, 330°C and 350°C, respectively. It is found that the Avrami-Eroféev model describes solid-phase changes in the oxygen absorption process adequately. The results show that the distributed activation energies of the oxidation process obtained by the Avrami-Eroféev model are 42.079 kJ mol−1, 42.944 kJ mol−1 and 41.711 kJ mol−1 for the YBaCo4O7+δ, Y0.95Ti0.05BaCo4O7+δ and Y0.5Dy0.5BaCo4O7+δ oxygen carriers, respectively. The kinetic model was obtained to predict the oxygen carrier conversion of oxygen absorption for different time durations. The kinetic parameters obtained here are quite vital when this material is used in reactors.

Process simulation and thermodynamic evaluation for chemical looping air separation using fluidized bed reactors

Chemical looping air separation (CLAS) is considered as a promising method for providing an efficient and economic oxygen supply for integration into oxy-fuel combustion power plants. This study is the first to develop a process model, identify the process characteristics, and optimize the thermodynamic behavior for CLAS processes using fluidized bed reactors. In the process developed model, a fast fluidized bed model and a bubbling fluidized bed model are used to respectively represent the oxidation reactor and the reduction reactor, while fluidized bed hydrodynamics and oxygen carrier redox reaction kinetics are considered to grasp the unique characteristics of CLAS process. The effects of operating parameters on CLAS operation are identified and observed that high reduction temperature and high fluidizing gas flow rate can enhance oxygen carrier conversion and reduce energy penalty in the reduction reactor. Multi-variable optimization to minimize specified energy consumption illustrates that, 38.1% energy savings can be achieved and the optimal oxidation operating temperature, reduction operating temperature, air flow rate, and fluidizing gas flow rate are determined as 830 °C, 950 °C, 1133.7 L/h, and 58.4 L/h, respectively. These results are valuable for use by engineers to optimize the process design and achieve energy-efficient operation for CLAS processes.

Enhanced performance of chemical looping combustion of methane with Fe2O3/Al2O3/TiO2 oxygen carrier.

Iron-based oxygen carriers supported on alumina or alumina/titania were prepared and evaluated for chemical looping combustion of methane. The reduction conversion of Fe2O3/Al2O3 and Fe2O3/Al2O3/TiO2 particles was markedly increased with increasing inlet concentration and was slightly enhanced by elevated operating temperatures. According to the shrinking core model, the mass transfer coefficients (kg) of Fe2O3/Al2O3 and Fe2O3/Al2O3/TiO2 reduction with methane are found to be 0.07 and 0.12 mm s−1. Complete combustion of methane is almost achieved for experiments conducted with Fe2O3/Al2O3 and Fe2O3/Al2O3/TiO2 operated as the Fe2O3/CH4 molar ratio reached about 5.4 and 4.4, respectively. Carbon deposition during methane combustion was avoided by using Fe2O3/Al2O3/TiO2 as an oxygen carrier. More heat was generated for the combustion of methane by Fe2O3/Al2O3/TiO2 oxygen carriers because methane more fully reacted with the Fe2O3 contained in the Fe2O3/Al2O3/TiO2 oxygen carriers.

CLOU process performance with a Cu-Mn oxygen carrier in the combustion of different types of coal with CO2 capture

The Chemical Looping with Oxygen Uncoupling (CLOU) process is a Chemical Looping Combustion (CLC) technology that allows the combustion of solid fuels with inherent CO2 separation by using oxygen carriers based on metal oxides. This technology has a low energy penalty and thus low CO2 capture costs. The oxygen carrier used in the CLOU process must be able to release gaseous oxygen, an aspect that limits the availability of metal oxides for this process. This work investigated the suitability of an oxygen carrier containing 34 wt% CuO and 66 wt% Mn3O4 (active phase Cu1.5Mn1.5O4) prepared by granulation regarding the CO2 capture, combustion efficiency and lifetime of the particles. The effect of the different types of coal (two sub-bituminous and a lignite) on combustion and CO2 capture efficiencies by CLOU was studied at different oxygen carrier to coal ratios in a continuous 1.5 kWth rig. It was found that full combustion could be reached regardless of the coal used. However, CO2 capture efficiencies were highly determined by coal rank. Finally, it was found that working with oxygen carrier to coal ratios higher than ϕ = 4, which corresponded to values of the variation of the oxygen carrier conversion lower than ΔXoc = 0.25, decreased the effect of chemical stress on the attrition rate. Therefore, it is clearly beneficial for the lifetime of oxygen carrier particles to operate with low variations of the oxygen carrier conversion (ΔXoc) between fuel and air reactors.

Copper-based oxygen carriers supported with alumina/lime for the chemical looping conversion of gaseous fuels

Copper (II) oxide in varying ratios was combined with either an alumina-based cement (Al300), or CaO derived from limestone as support material in a mechanical pelletiser. This production method was used to investigate its influence on possible mechanical and chemical improvements for oxygen carriers in chemical looping processes. These materials were tested in a lab-scale fluidised bed with CO or CH4 as a reducing gas at 950 °C. As expected, the oxygen carriers containing a greater ratio of support material exhibited an enhanced crushing strength. Oxygen carriers comprised of a 1:3 ratio of support material to active CuO exhibited increased crushing strength by a minimum of 280% compared to pure CuO pellets. All oxygen carriers exhibited a high CO conversion yield and were fully reducible from CuO to Cu. For the initial redox cycle, Al300-supported oxygen carriers showed the highest fuel and oxygen carrier conversion. The general trend observed was a decline in conversion with an increasing number of redox cycles. In the case of CaO-supported oxygen carriers, all but one of the oxygen carriers suffered agglomeration. The agglomeration was more severe in carriers with higher ratios of CuO. Oxygen carrier Cu25Al75 (75 wt% aluminate cement and 25 wt% CuO), which did not suffer from agglomeration, showed the highest attrition with a loss of approximately 8% of its initial mass over 25 redox cycles. The reducibility of the oxygen carriers was limited with CH4 in comparison to CO. CH4 conversion were 15%–25% and 50% for Cu25Ca75 (25 wt% CuO and 75 wt% CaO) and Cu25Al75, respectively. Cu25Ca75 demonstrated improved conversion, whereas Cu25Al75 exhibited a trending decrease in conversion with increasing redox cycles.